In recent years, OFDM technology has become the mainstream in wireless communication physical layer technologies, due to its effective resistance to multi-path interference and narrow-band interference, high spectral efficiency and high transmission rate. OFDM is adopted as the physical layer technologies for providing multi address access (OFDMA) no matter in subsequent evolution of 3G (such as LTE) or in wireless broadband access technologies popular at present (such as IEEE 802.16 and IEEE 802.20).
In OFDMA system, it is divided as plural OFDM symbols in time domain, while it is divided as plural sub-channels in frequency domain, and each sub-channel is a set of a group of sub-carriers. Generally, time frequency area formed by crossing of one sub-channel and one or several symbols is called slot, which is the smallest allocation unit of OFDMA system. In such way, the physical layer resources of OFDMA frame can be logically represented by a two-dimensional rectangular table formed by slot and sub-channel. As shown in FIG. 1, a grid is a slot and the physical layer resources allocated for the terminal are resource blocks in the unit of slot, which are generally blocks similar to rectangle (such as IEEE 802.16) with a two-dimensional time-frequency structure. According to different service requirements and different modulation coding methods determined by AMC (Adaptive Modulation and Coding) performed upon the channel conditions, and the dispatching of resource blocks allocated thereto in different times may greatly differ, that is, the number of carriers allocated to the terminal may differ after each dispatching, which may lead to great change of carrier number the terminal has in different times.
As in OFDMA system the terminal usually has the restriction on maximum total transmit power, it only needs to satisfy that the summation of transmit power of all carriers allocated at present is smaller than or equal to the maximum total transmit power restriction, in order to save power resources, increase uplink coverage and enhance channelized gain. At present most of products adopt the above method. Additionally, the restriction of maximum total transmit power restriction of the terminal can also be allocated to the most possible carriers number of the terminal as the of the carrier maximum transmit power restriction, which however will lose channelized gain and seriously waste power resources.
When the power constraint is merely to require the power summation of carriers allocated at present is smaller than the total transmit power of the terminal, following problems exist: in OFDMA system, after AMC determines the modulation coding method according to channel conditions and power control determines the carrier's transmit power according to the modulation coding method, the total transmit power of the terminal will change significantly resulting from the significantly change of carrier number allocated to the terminal by dispatching. The traditional method at present is as follows: AMC and power control are performed before the dispatching and they are conducted separately. The object of AMC and power control is each carrier. As stated in the above, when the upper limit of carrier's transmit power is used in the power control to restrict the uplink transmit power, the maximum transmit power of the carrier can only be determined according to bandwidth allocation condition of the previous one frame's or prior dispatching. As dispatching has not been performed at this moment, it is unknown how much bandwidth will be allocated to MS at this time. The bandwidth allocated to the terminal by the dispatching according to modulation and coding method and QoS (Quality of Service) may either be less or more than the previous one frame. If it is less than the previous allocation, excessive restriction may be applied to carrier power in the power control, wasting the power resources; if it is more than the previous allocation, the carrier's maximum power restriction in the power control will become meaningless, where, the calculated carrier transmit power summation may be greater than the maximum total transmit power of the terminal, however the terminal actually can not transmit so much power, resulting in that the terminal allocates all the power to all its carriers, however, carrier power is still much smaller than the set value and power spectrum density is suddenly decreased without control, that is, carrier power is suddenly decreased without control. In this way, the carrier's actual transmit power will be much smaller than the carrier transmit power determined in the power control and the obtained SNR (Signal to Noise Ratio) will be small, which can not meet the demand of its modulation coding method determined in AMC and finally leads to a large amount of error codes. For example, in OFDMA system of IEEE 802.16e with bandwidth of 10 MHz and FFT of 1024 point, each sub-channel of uplink comprises 24 physical sub-carriers, the maximum total transmit power of one terminal is PMAX, the modulation coding method determined by AMC in the last time is MC1, the carrier transmit power determined in power control is P2, the bandwidth allocated in the dispatching is N1 sub-channels, single carrier power is P1 satisfying N1×24×P1≦PMAX; the modulation coding method is first determined to be MC2 according to SNR in AMC this time, power control determines the carrier's transmit power to be P2. To guarantee the validity of this power, the magnitude of the carrier power will be restricted in power control to make sure that the summation of carrier power is smaller than the maximum transmit power of the terminal. However, the dispatching has not been performed at this moment, the system does not know how much bandwidth is allocated to the terminal at this time and only can perform the maximum power restriction according to bandwidth allocation of the last time, therefore the maximum power restriction here becomes meaningless. Thereafter, the dispatching allocates the bandwidth of N2 sub-channels, wherein N2 may be either greater or smaller than N1, and at this moment it is possible N2×24×P2≦PMAX or N2×24×P2≧PMAX. In the case of N2×24×P2≦PMAX, it is possible that the terminal's maximum transmit power restriction has no influence on carrier power or that it restricts terminal carrier's transmit power when be as the carrier's maximum power restriction in power control, leading to that the terminal requiring higher power can not obtain the higher transmit power that would have been obtained and rendering waste of power resources. In the case of N2×24×P2>PMAX, the terminal actually can not transmit the carrier's transmit power set in the power control and the carrier's transmit power will suddenly become lower than the value set in the power control, which leads to that SNR (SNR related here specially refers to ratio of signal to power of interference or noise) can not satisfy the requirement of modulation coding method previously set in the AMC and a large amount of error codes occur. In an easier condition, after two dispatching, the modulation coding method and carrier's transmit power set are the same, while the bandwidth allocations are different, that is, MC1=MC2, P1=P2 and N1 is not equal to N2. There is no problem in case of N2<N1 and N2×24×P2≦PMAX, however, if N1<N2, N2×24×P2>PMAX may be occur and the carrier's actual transmit power of the terminal must be smaller than P2. However, in power control P2 is determined according to modulation coding method, in this way, SNR can not satisfy the requirement of modulation coding method determined previously and a large amount of error codes will occur.
Time-Frequency two-dimensional resources bring about many advantages to OFDMA, such as flexibility, but also non-controllability and waste of resources. The existing systems all adopt traditional method that AMC is first performed, then power control and at last the dispatching. However, this will deteriorate the performance of OFDMA system and make the above problem unsolvable.
From searches on academic theses and patent documents and survey of research institutions and companies engaging research of OFDMA system, it is found out that the above problem is seldom focused on and the corresponding solutions are not perfect enough.